The invention relates to a slope control device for an electric data transmission system comprising a first line and a second line for differentially transmitting binary data pulses in such a manner that a first logic value of the data pulses has a high potential on the first line and a low potential on the second line associated therewith, and a second logic value of the data pulses has a low potential on the first line and a high potential on the second line associated therewith.
Such a data transmission system usually includes a larger number of data locations that are interconnected by means of the two lines and of which at least part is capable of operating both as transmitter and as receiver.
A first advantage of such differential transmission via two lines consists in that interference pulses reaching both lines in equal manner are eliminated in the differential assessment or decoding of the data pulses transmitted via the lines. A second advantage consists in that such a differentially operating data transmission system displays redundancy with respect to a number of line errors, so that error-free transmission thus is still possible if the two lines are shorted to each other if one of the two lines is open or if one of the two lines is shorted to a ground potential or a supply potential.
More detailed explanations in this respect can be found in DE 195 23 031 A1.
Such a differentially operating data transmission system can be a CAN system. The term CAN stands for Controller Area Network. Closer details in this respect can be found in the book xe2x80x9cController Area Network: CANxe2x80x9d by Konrad Etschberger, Carl Hanser Publishing House 1994, ISBN No. 3-446-17596-2.
Such CAN systems are employed for example in the field of motor vehicles. With such differentially operating data transmission systems, an error detection and error processing circuit may be employed through which, in evaluating or decoding received data, switching over to different signal selection is possible depending on the line condition. In case both lines of the data transmission system are deemed error-free, differential signal evaluation takes place such that the potentials on both lines are compared to each other. In case a line error is found for one of the two lines, it is switched over to a mode of operation in which the potential patterns or curves of the line deemed error-free only are evaluated. One may proceed in corresponding manner if it is found out that the two lines are shorted to each other.
Such circuit arrangements for error recognition, error processing and switching over to different evaluation modes are known from the afore-mentioned DE 195 23 031 A1 and from the applicant""s own German patent applications 198 26 388 and 198 50 672.
With such a differentially operating data transmission system, the potential transitions on both lines, which occur simultaneously, should have the same slope steepness in order to keep electromagnetic interference radiation low and to be able to obtain good error elimination in the receiving data locations. Conventional attempts to obtain this consist in associating a slope regulating circuit in each data location of each line, which regulates the slopes on the particular line to a predetermined desired slope steepness value, with the desired slope steepness values for both lines being identical. Each control loop, however, has its own amplification, its own phase compensation and its own offset behavior. Due to the fact that the control loops of different slope control circuits differ from each other in these parameters in practical embodiments, there are corresponding slope steepness deviations arising in the output signals.
It is the object of the invention to overcome the foregoing disadvantages and to provide for slope steepness conformity on both lines.
According to the invention, this is achieved by way of a slope control device according to claim 1, which may be developed further in accordance with the dependent claims.
In accordance with an embodiment of the invention, in the normal case, i.e., with correct line conditions, slope steepness regulation is effected only with respect to the potential curve of one of the two lines. The potential curves on the two lines are compared with each other with respect to the slope steepness thereof and the slope steepness of the potential curve of each line is controlled without slope steepness regulation in accordance with the comparison result.
Due to these measures, the slope steepness of the potential curve of one of the two lines is regulated to the desired nominal value, and the slope steepness of the potential curve of the other line is subjected to corresponding adjustment control.
In practical embodiments, each line in each data location has a slope control circuit, with the slope control circuit of a line carrying out slope regulation and the slope control circuit of the other line carrying out adjustment control to the regulated slope steepness of the other line. In this respect, the slope-steepness regulating slope control circuit constitutes a master circuit whereas the adjustment-controlling slope control circuit constitutes a slave circuit. As regards the allocation of master circuit and slave circuit, it is possible to use either a fixed assignment or a switchable assignment in which each of the two slope control circuits can be selectively switched to a regulating mode or an adjustment control mode of operation, and in doing so, it may be selected freely which one of the two slope control circuits is switched to the regulating mode and which one of the two slope control circuits is switched to the adjustment control mode.
The latter alternative is to be preferred if the slope control device according to the invention cooperates with a line error checking circuit by means of which the two lines can be checked with respect to the presence of correct line conditions or erroneous line conditions. In case an error-free condition has been ascertained for both lines, it is possible then to switch an arbitrary or predetermined one of the two slope control circuits to the regulating mode and the other slope control circuit to the adjustment control mode. If the line checking circuit ascertains a line error for one of the two lines, the slope control circuit of the intact, other line is switched to the regulating mode.
In a preferred embodiment of the slope control device according to the invention, each of the two lines comprises at least one switchable potential switching means in the form of a controllable analog switch means for data pulse generation, and a slope control circuit for controlling the slope steepness of the data pulses. The analog switch means operates, in the on-state and thus in the range of potential curve slopes in an analog mode, for example as analog amplifier, and in states in which it is turned off, it represents a pure switch behavior. Employed as such an analog switch means is, for example, a transistor, preferably in the form of a MOS transistor which, in the on-state, can be controlled in analog manner via its control electrode.
For comparison of the slope steepness of the potential curve of one line to the slope steepness of the potential curve of the other line, there may be used a differentiating circuit by means of which the potential curves on both lines can be differentiated each, as well as a comparison circuit by means of which the resulting differential signals may be compared with each other. In doing so, the slope control circuit that is in the adjustment control mode is controlled by the output signal of the comparison circuit.
Due to the fact that, in a slope control device according to an embodiment of the invention, only the slopes of the potential curve on one line are still regulated and the slope steepness of the potential curve on the other line is adjustment-controlled in accordance with the actual deviation state of the slope steepnesses on both lines, there is always an identical slope steepness obtained on both lines, at least as long as both lines are error-free.